The invention relates to an apparatus for the analysis of print control fields and in particular offset printing.
At the present time the printing process is primarily controlled by printing control fields which usually are analyzed densitometrically or more recently even colorimetrically, to obtain control variables for the setting and regulation of the printing machine or other relevant information for the printer. In offset printing, in addition to various other control fields, in particular single color solid-tone fields and single color half-tone fields, for all of the colors involved in the printing process, two-color and sometimes even three-color overprinted solid-tone fields are also employed. In the case of single color solid-tone fields the relevant measuring variables are the layer thicknesses of the printing inks involved. The layer thicknesses are determined by the densitometric color densities. In the case of half-tone fields, primarily the dot area in the print or often the dot gain in the basic half-tone film are determined. In the case of overprinted fields, usually the so-called trays of the second (or third) down ink on the first (or second) down ink (inks) is determined.
For the densitometric analysis of such print control fields and the determination of the variables relevant for the printer and for the control and regulation of the printing process, a series of densitometers has been available for a long period of time. Densitometers range from relatively simple manual devices operating off-line through table densitometers (scanning densitometers), to on-line machine densitometers mounted directly on the printing machine, which at the present time are mostly computer controlled and thus are efficient and simplistic in operation. The best known representatives of advanced manual densitometers include the devices with the designation series D183, D185 and D186 of the Gretag AG Co. in Regensdorf, Switzerland.
A characteristic of the practical operation of such manual densitometers is that the operator must position the densitometer on the control fields of interest and, via control elements, manually indicate to the device which of the variables are to be determined and displayed. Many of these devices are already capable of recognizing and displaying the color of the control field (i.e., for example, whether a cyan, magenta, yellow or black field is involved), automatically by certain criteria. However, these devices must still be instructed whether the color density or the dot area or the ink trap is to be determined and displayed and the various functions of the device must therefore still be selected by the operator. A densitometer capable of automatically recognizing the type of the control field being examined and automatically setting its measuring variables would significantly enhance the ease of operating such a device.
In EP-A-O 283 899 (corresponding to U.S. patent application Ser. No. 307,735 of Mar. 25, 1987; U.S. Pat. No. 4,947,348) a manual densitometer is described, which is equipped with such an automatic operating mode or function switch and is capable of automatically recognizing and distinguishing a limited set of control field types and of determining and displaying values characteristic of each individual control field type. The recognizable control field types include single color solid-tone fields, single color half-tone fields and two-color overprinted solid-tone fields. It is further automatically determined whether the instantaneous measurement is being carried out at an unprinted location of the sheet. The device determines in each measuring position the color density in all available measuring channels (usually red, blue, green and visual, corresponding to the ink densities of cyan, yellow, magenta and black) and determines by comparison with given color density reference values the type of the control fields involved, the color present, etc. The device then calculates the variable associated with the control field type and displays it. For the computation of certain complex variables such as, for example, the ink trap and dot area, additional measured values from other types of control fields, (e.g. solid-tone densities of the colors involved) are required. In such cases, the device indicates to the user by appropriate displays that other measurements must be carried out and displays the complex variables only after all necessary additional measurements have been carried out in proper sequence.
The densitometer described in EP-A-O 283 899 already offers a more simplified operation relative to devices not equipped with such an automatic functional switch, in that the user does not have to be concerned with the specific functional setting of the device for the control fields involved and is able to base more complex measurements on automatic user guidance. However, in view of the distinguishing criteria selected (comparison with given constant color density reference values) the reliable recognition of different types of color fields may be subject to problems, at least in certain extreme situations. Thus, for example, it is difficult to reliably distinguish solid-tone fields and half-tone fields over the full dot area range over the entire solid-tone range. The recognition of the colors of the control fields is also not optimal. Further, the device is not able to distinguish half-tone fields of different nominal dot area, such as those frequently used in the same print control strip.
Finally, in the case of half-tone fields, while the device displays the dot area, it is not able to determine and display the dot gain relative to the dot area values in the half-tone film, which is often desirable.